In a general wireless communication system, typically, a single carrier is considered in uplink and downlink although different bandwidths are set for uplink and downlink. For example, it is possible to provide a wireless communication system based on a single carrier in which the number of carriers constituting each of the uplink and the downlink is 1 and bandwidths of the uplink and the downlink are symmetrical to each other.
The international telecommunication union (ITU) requires that candidate technologies for IMT-Advanced support a bandwidth extended compared to a conventional wireless communication system. However, it is difficult to allocate frequencies of large bandwidths throughout the world, except for some regions. Thus, as a technology for efficiently using small fragmented bands, a carrier aggregation technology which is also referred to as bandwidth aggregation or spectrum aggregation has been developed to allow a number of physical bands to be combined in the frequency domain to be used as a large logical band.
Carrier aggregation has been introduced in order to support increased throughput, to prevent cost increase due to introduction of broadband RF elements and to guarantee compatibility with existing systems. Carrier aggregation enables data exchange between a UE and an eNB through a plurality of groups of bandwidth-based carriers, which are defined in a conventional wireless access system (for example, in the LTE system in the case of the LTE-A system or in the IEEE 802.16e system in the case of the IEEE 802.16m system).
Here, bandwidth-based carriers defined in the conventional wireless communication system may be referred to as component carriers (CCs). Carrier aggregation technologies may include, for example, a technology that combines up to 5 CCs to support system bandwidths of up to 100 MHz even though a single CC supports a bandwidth of 5 MHz, 10 MHz or 20 MHz.
When such carrier aggregation technology is used, data can be simultaneously transmitted or received through a number of uplink/downlink CCs. Accordingly, the UE may monitor and measure all CCs.